This is due to the small Al2O3 grain size in the sample microstructures which resulted from the presence of MgO . A study conducted by Coble revealed that the role of MgO is to accelerate the sintering rate for Al2O3, thus hindering Al2O3 grains from growing. The critical condition for Al2O3 to undergo discontinuous grain growth was averted by the enhancement of the sintering rate by the presence of MgO [7]. It is clearly shown that the effect of MgO particle size plays an important role in the development of the hardness value and microstructures of the samples.